Abstract: FSO systems rely on line-of-sight, and thus can be easily impaired due to disruptions such as atmospheric turbulence. There is a need for a more robust communication system allowing longer distances to be bridged and/or downtime to be reduced. An optical transmitter is provided generating at least one optical alignment beam. The transmitter comprises at least one alignment modulator, to modulate the alignment beam with transmitter directional data. A suitable receiver may demodulate this information and use the directional data from the transmitter to simplify the attainment and/or maintenance of a sufficient degree of alignment. Additionally or alternatively, the at least one alignment beam may be used to detect, characterize and/or monitor one or more environmental parameters.

Abstract: In an optical transmission system a transmitter (2) generates a sequence of time domain division multiplexed pulses and transmits them via an optical transmission medium (4) to a receiver (6). In the receiver (6) the received optical multiplex signal is applied to an optical demultiplexer (30) and to a clock recovery element (48). The clock recovery element (48) generates optical clock pulses which are used to control optically controlled optical switches (32 . . . 38) which demultiplex the optical signal. The optical clock recovery circuit is based on a laser which is mode locked by the received optical signal. The mode locked laser comprises an optical amplifier (50) having its output coupled to its input by a feedback loop comprising a polarization controller (52), a saturable absorber (61), a coupling element (56) and a adjustable delay (51). The received optical signal is coupled into the feedback loop by means of the coupling element (56).

Abstract: The invention relates to an optical unit (17) for synchronizing clock signals. The unit (17) comprises at least two ring lasers (1', 1"), each ring laser (1', 1") generating a repetitive optical pattern at a different repetition frequency f.sub.i. The repetition frequency of at least one of the ring lasers (1', 1") is variable. The unit (17) comprises detection means (19) adapted to simultaneously receive the optical patterns from two ring lasers (1', 1") to be synchronized with each other and to compare these patterns. The ring laser, whose repetition frequency is variable, is controllable on the basis of a signal from the detection means, by which the optical path length of this laser is changed. The invention also relates to a high-frequency carrier transmission system comprising an optical unit as described hereinbefore.

Abstract: An optical unit for multiplying the frequency of a clock signal includes at least two series-arranged ring lasers. Each ring laser has a different resonance frequency f.sub.i. The unit has an input for receiving a signal, for example, a low-frequency electric signal for modulating one of the ring lasers. The repetition frequency of at least one of the ring lasers is variable by adapting the optical path length of the resonator.

Abstract: An optical unit for restoring a pulse pattern is described. The unit has an input for receiving a signal pulse series at a modulation period T and wavelength .lambda.hd p and comprises a pulsed laser for supplying a pulse series whose distance between two consecutive pulses is equal to n.T, in which n is an integer and is variable throughout the pulse series. Pulses of the signal pulse series can be injected into the pulsed laser. Moreover, the unit comprises at least a detection system arranged in the path of the radiation emitted by the pulsed laser, which system detects on the basis of a variation of a feedback-sensitive parameter of the laser.

Abstract: In a transmitter in a coherence multiplexed transmission system the output signal of a laser is directly applied to a channel and indirectly via a plurality of cascaded circuits of delay elements and modulators. The modulators vary a component of the autocorrelation function of the output signal of the transmitter as a function of an associated modulation signal (m.sub.1, m.sub.2, m.sub.3). In the receiver the autocorrelation functions values of the channel output signal for delays (D.sub.1, D.sub.2, D.sub.3) are determined in the demodulators. These autocorrelation function values from the modulated signal (m.sub.1 ', m.sub.2 ', m.sub.3 '). To reduce the number of periodic components of the autocorrelation function of the signal source, and hence enhance the possible transmission capacity, the transmitter comprises a decorrelation modulator for modulating the signal source with pulses generated by a pulse generator.

Abstract: The invention relates to an optical unit for generating an optical data signal in conformity with a data pattern. The unit has an input for receiving the data pattern with a modulation period T, a pulsed laser for supplying an optical pulse series having a pulse period n.T and a pulse duration .tau., in which n is an integer, and a converting unit for converting this pulse series into an optical data signal in conformity with the data pattern. The converting unit is a radiation-controlled optical switch. This switch has a first input for receiving the data pattern in the form of an optical pulse series whose pulses have a pulse width b, a second input for receiving the optical pulse series from the pulsed laser for sampling the data pattern during periods when the switch is open, and an output for supplying a data pulse series having a pulse width .epsilon..b, in which 0 <.epsilon.<1.

Abstract: A unit for generating signal pulses. comprises a first, pulsed laser for supplying a pulse series having a pulse period T and a wavelength .lambda..sub.1, and a modulation unit comprising a second laser. The second laser has a wavelength band which is different from .lambda..sub.1 and can be modulated at a modulation period T in accordance with a data signal to be transported. The radiation from the second laser ) can be injected into the first laser at instants for which it holds that E(P.sub.m)>E(LP.sub.i), in which E(P.sub.m) is the radiation energy of the second laser injected into the first laser at the relevant instant and E(LP.sub.i) is the radiation energy built up in the first laser at the relevant instant. Moreover, the unit comprises a wavelength discriminator which selects the pulses of wavelength .lambda..sub.1 from the radiation emitted by the first laser after injection.

Abstract: An optical signal-regenerating unit for a received optical signal pulse series (S.sub.1) having a modulation period T and a wavelength .lambda..sub.d. The optical unit includes a pulsed laser for producing a regenerated pulse series S.sub.2 having a pulse period equal to the modulation period T. Pulses of the received pulse series S.sub.1 are injected into the pulsed laser at instants at which the energy of the injected pulse exceeds the energy of a pulse then being built up in the laser. By maintaining that relationship, the multimode spectrum of the pulse laser is converted into a single mode spectrum having a wavelength equal to the wavelength .lambda..sub.d of the received radiation.

Abstract: In an optical soliton transmission system an optical transmitter (2) generating short pulses is coupled to a glass fiber cable (4) comprising sections of glass fiber (7, 9, 11, 13, 15) with interspaced semiconductor laser amplifiers (8, 10, 12, 14). An optical receiver (6) is connected to the end of the glass fiber cable (15). In optical soliton transmission Erbium doped fiber amplifiers and dispersion shifted fiber are currently used in the 1.5 .mu.m wavelength region. However, by utilizing standard fiber in the 1.3 .mu.m wavelength region it is possible to use a lower soliton power compared with a state-of the-art transmission system.

Abstract: The invention relates to an optical switch (1) which is switchable by means of light. The switch (1) comprises an optical wave-guiding structure (3) having an entrance (5) to which a signal can be applied and an exit (7). The refractive index of the wave-guiding structure (30) is variable by means of light intensity. The switch (1) further comprises a radiation source unit (15) whose radiation can be injected into the wave-guiding structure (3). The radiation source unit (15) comprises a pulsed laser (17) having a repetition time T and a pulse duration p and a medium (19) for transporting the radiation supplied by the laser (17). The radiation source unit (15) is provided with setting means (21 ) for stabilizing the wavelength of the laser (17) at a number n of selected wavelength bands from a number N of possibly selectable wavelength bands within one and the same pulse supplied by the laser (17). Moreover, the medium (19) is dispersive at a wavelength-dependent travel time t.sub.